The invention relates to novel crystalline forms of 3-(4-amino-l-oxo-l,3 dihydro-isoindol-2-yl)-piperidine-2,6-dione, including a novel urea cocrystal of 3-(4-amino-l-oxo-l,3 dihydro-isoindol-2-yl)-piperidine-2,6-dione; a novel gallic acid cocrystal of 3-(4-amino-l-oxo-1,3 dihydro-isoindol-2-yl)-piperidine-2,6-dione; a novel propyl gallate cocrystal of 3-(4-amino-l-oxo-1,3 dihydro-isoindol-2-yl)-piperidine-2,6-dione; and a novel L-tartaric acid cocrystal of 3-(4-amino-1-oxo-1,3 dihydro-isoindol-2-yl)-piperidine-2,6-dione. The preparation and characterization of the novel crystalline forms according to various embodiments of the invention are described. The invention also relates to pharmaceutical compositions containing the novel crystalline forms, and the therapeutic use of the novel crystalline forms.